It is known in the art to produce flexible sheet-type products, particularly suitable for use as sound insulation, heat insulation and the like, by incorporating a dense, particulate filler, such as barium sulfate (barytes), barium titanate, silicon dioxide (silica) and the like in rubber compositions. However, the high specific gravity, for example, 4.54 for barytes, causes such products to be quite heavy. In order to overcome this problem and produce a lightweight material, it is also known to incorporate hollow microspheres, such as hollow glass or silicate spheres, film forming plastics, etc. of low density in certain polymer products. While sheet-type structures incorporating these hollow microspheres produce a lighter sheet-type structure, problems in the preparation of the sheet-type structure make it impossible to obtain the full benefit of the use of the lightweight material. Specifically, conventional practice is to incorporate the hollow microspheres in a liquid polymerizable material before polymerization. In such cases, however, at least some degree of mixing is necessary to produce a homogeneous mixture which can be formed into a sheet-type structure. Such mixing results in the fracture or crushing of the hollow microspheres and thus a loss of the benefit of the microspheres. It has been found that this problem, of the crushing of the hollow microspheres, is particularly troublesome in attempts to incorporate hollow microspheres into a rubber composition, since the formation of rubber compositions requires high shear forces during mixing operations. The ultimate result is that the hollow microspheres are crushed during mixing with a substantial loss if not the loss of all of the benefit of the hollow microspheres.